In general, an electro-explosive device (EED) receives electrical energy and initiates a mechanical shock wave and/or an exothermic reaction, such as combustion, deflagration, or detonation. EEDs have been used in both commercial and government applications for a variety of purposes, such as to initiate the inflation of airbags in automobiles or to activate an energy source in an ordnance system.
Prior art EEDs include those that use a bridgewire to ignite an ordnance material. A bridgewire is a thin resistive wire attached between two contacts. The ordnance material surrounds the bridgewire. When current is passed through the bridgewire ohmic heating results. When the bridgewire reaches the ignition temperature of the ordnance material, the ordnance material initiates. Typically, the ordnance material is a primary or pyrotechnic charge which ignites a secondary charge, which in turn ignites a main charge. EEDs that use a bridgewire have significant disadvantages in modem applications. For example, EEDs are subjected to increasing levels of electromagnetic interference (EMI) in many military and civilian applications. High levels of EMI present a serious danger because the EMI may couple electromagnetic energy through a direct or indirect path to an EED, causing it to fire unintentionally. EEDs may also be unintentionally fired by electrostatic discharge (ESD). Conventional devices to protect against unintentional discharge, such as passive filter circuits and EMI shielding, present their own space and weight problems in typical applications.
In order to reduce the sensitivity of an EED to stray signals, the total energy of the firing signal which is necessary to ignite the EED may be increased. As a result, low level stray signals may be conducted through the bridgewire without causing any ignition and only the higher level firing signal would have sufficient energy to ignite the EED. A higher magnitude firing signal, however, is not always desirable. In many applications, such as in automobile airbags, available power is severely limited, making it necessary to provide an EED that has a low firing energy, which may be near the energy level of potential spurious signals such as those from ESD or EMI sources.
One type of EED that alleviates some problems with accidental firing is called a semiconductor bridge, or SCB. An SCB may use less energy than that used by a bridgewire EED for the same no-fire level. For example, the energy required by an SCB may be an order of magnitude less than that requited by a bridgewire device with the same no-fire performance. An SCB is a ordnance material initiating device built on a semiconductor substrate. The SCB typically ignites the ordnance material with a hot plasma. When the SCB fires, it creates a high temperature plasma (for example, greater than 4000 K in some cases) with high power density that ignites the ordnance material. The SCB may generate plasma in less than several microseconds as compared to the bridgewire, which may heat to the point of initiation in hundreds of microseconds. The ordnance material ignited by the SCB is typically an adjacent ordnance material or primary explosive that is ignited in a matter of microseconds and in turn ignites an output charge. The excellent heat transfer characteristics of the semiconductor provide a high capacity heat sink for the SCB and thus a relatively high no-fire level. Generally an SCB should be driven by a low impedance voltage source or a capacitive discharge to properly support an avalanche condition that results in plasma creation.
The use of EEDs in automobile airbags and other safety critical applications presents several problems in addition to the prevention of unintentional firing. For example, the reliability of an airbag EED is critical. The airbag EED must fire reliably, and must be manufactured in a way that allows some verification of reliability. Conventional SCBs have some disadvantages that make it difficult to produce verifiably reliable SCB EEDs. For example, SCBs provide a very hot but low energy ignition source that lasts only for microseconds. In typical SCBs the amount of energy output is dependent upon, and is less than, the level of energy input. In cases in which only a very small amount of output energy can be produced, the output energy may not be sufficient to provide reliable ignition.
Reliability of conventional SCB components is also difficult to verify. One reason for this is that in conventional SCBs, the ordnance material and the SCB must be tightly coupled in order to transmit the small energy output of the SCB to the primary ordnance material. That is, at the ordnance material/SCB interface the ordnance material must be in intimate contact with the SCB at all times for SCB firing to reliably ignite the ordnance material. Test methods have been developed to attempt to verify the ordnance material/SCB interface in bridgewire devices but these test methods, generally do not work well for semiconductor devices. For example, it may be possible to verify the presence of the proper amount of ordnance material by weighing, but it is very difficult to verify a proper interface, or intimate contact between the SCB and the ordnance material. Even if a proper interface exists at manufacture, it is difficult to determine whether an interface in a particular device is degraded over time, for example by vibration or shock. Even given a proper interface, without positive retention of the SCB against the ordnance material, the ordnance material may be thrown off by the shock generated by the SCB firing, rather than ignited. Positive retention introduces its own problems, however, including added cost and complexity without resolving verification of continued reliability in the field. In addition, the forces applied to the SCB in positive retention may break the SCB and/or connection bonds in the device.